The trail running from Mt. Rausu-dake (羅臼岳, Rausu-dake) across to Mt. Io-zan (硫黄山, Iou-zan) is the only well-trodden traverse along Shiretoko Peninsula (知床半島, Shiretoko-hantou), jutting out from Hokkaido’s northeast corner. It's an epic walk, and a surprisingly seldom-visited at that. Here’s we’ll be covering a hike starting at Kinoshita Lodge (木下小屋, Kinoshita-goya) at Iwaobetsu Onsen (岩尾別温泉), stopping for the night at Mitsu-mine (三ッ峰) campsite, summitting Io-zan the following day, and descending to Kamuiwakka (カムイワッカ) waterfall the following afternoon.
Iwaobetsu Onsen features both a hotel and a backcountry-style lodge (just up the paved road from the hotel). If you can swing it, it's a great idea to stay the night before the hike either at Kinoshita Lodge or at the onsen hotel. Alternately, a cheaper stay might be found at nearby Utoro. Either option allows for a good early start.
The first half of the trail up to Rausu is rife with Mongolian oak and bears alike, leading up the broad spine of a ridge. Make sure, especially in denser, poor-visibility areas, to make your presence known so you don’t run across any bears.
At about 590 meters you’ll come to a rocky outcrop where you’ll be able to see down to the Shiretoko Goko (知床五湖, ‘Shiretoko Five Lakes’ district) at the foot of the mountain. After you’ve climbed a little further you’ll come to Yasakichi-mizu (弥三吉水), a small rest area with a reliable water source—the water coming out of the pipe here can be drunk untreated, and tastes terrific. Past here the trail will level out a bit along Gokuraku Plateau (極楽平, Gokuraku-daira), and you’ll continue through tunnels of twisted Erman’s birch.
Past Ginrei-sui (銀冷水), another water source, you’ll come to the toughest part of the climb up Rausu: Oo-sawa (大沢, lit. 'Big Ravine'). Prevailing winter winds deposit huge amounts of snow here, which can remain well into July. If you’re climbing in the first half of the summer, don't be surprised to see your fellow climbers donning crampons in their t-shirts and shorts. Past this trial by ice, you'll arrive at the broad Rausu Plateau (羅臼平, Rausu-daira), the saddle between Rausu and neighboring Mitsu-mine (三ツ峰). If you happen to climb after the snow melts, you’ll see a bright valley of yellow- flowered rhododendron, pincushion plant, Kamchatka rhododendron, and wedgeleaf primrose.
When you get to Rausu-daira, you’ll finally be able to cast your eyes over the rocky summit of Rausu-dake, as well as along the mountains stretching down the peninsula. It might be a good idea to leave your pack here before heading up to the summit (and picking it up on your way towards Io-zan). Make sure to store any food you’re carrying in the nearby food locker so it doesn’t get eaten by bears or foxes while you’re trekking above.
First you’ll cross a shallow grade covered in dwarf stone pine, then you’ll come to a steep pile of huge boulders up which you’ll climb, hand over foot, to the summit of Rausu-dake. On a clear day, you should be able to see all the way down the many-peaked range to Io-zan, your eventual goal.
Head back down to Rausu-daira and then climb the slope up to Mitsu-mine, the three-peaked massif just north east of Rausu-dake. The trail crosses a small pass between the peaks, but a faint trail leads up to the highest of the three separate summits. Past Mitsu-mine you’ll head down a ravine full of Aleutian avens to a narrow campground, where you'll be spending the night. As the space here is pretty narrow, be careful when setting up your tent so as not to crush any nearby plant life. Here too you’ll find a food locker for the food you’re carrying. Remember to pack out any trash.
The next day’s hike features plenty of ups and downs to wear you out, and the descent from Io-zan is no slouch, distance-wise, either. Gird your loins, and keep an eye on the weather as well, as the cirque leading around Minami-dake and Chienbetsu-dake can be extremely windy, and poor visibility around the summit of Io-zan could spell trouble.
The first big climb is Sashirui-dake (サシルイ岳). You’ll hike past fields of keyflower and more Aleutian avens as Mitsu-mine and Rausu rise behind you. Feel free to stop and take in the view behind you as often as you like. You’ll head around the west flank just below the summit and descend by a ravine, where you might find some leftover snow.
As the trail levels out from the descent you’ll cut left and start the trek up Mt. Okkabake-dake (オッカバケ岳). Okkabake, like Sashirui, is a reasonably short climb. The slope is characteristically thick with dwarf stone pine, so by the time you reach the summit you’ll probably find your skin and clothes smelling heavily of pine resin. Descending the far side, you’ll see Io-zan reflected in the blue waters of the twin-pond Futatsu-ike (二ッ池, lit. 'Two Ponds').
There is a campsite at Futatsu-ike. If you're filling up on water, remember to boil, filter, or treat the water before drinking it. You're sharing it with the bears.
The trail will wind around the lake’s edge and mount the western ridge of Minami-dake (南岳). From this ridge it’s about half an hour to the summit of Minami-dake proper. This ridge forms a part of the mountain ridgeline that hangs about Io-zan. On the scree slope around Minami-dake, you’ll find that the small white Shiretoko violet has taken root here.
Following the ridgeline from Minami-dake, you’ll drop temporarily onto the east side of the ridge and through an area of tall grass, following a ravine up the back of Mt. Chienbetsu-dake (知円別岳). Here is a junction that can take you along a ridgeline to Mt. Higashi-dake (東岳). Here too, just to the north, is the head of the deep Ubushinotta River (ウブシノッタ川, Ubushinotta-gawa) valley. The trail you want passes directly below the summit of Mt. Chienbetsu-dake on a traverse to the top of the Io-zan ridgeline.
Here you'll find yourself in truly volcanic country. You’ll cross a narrow ridge of white volcanic ash and come to an area characterized by tall cliffs, through which you’ll weave over the complex topography. As you wind about the outcrops, you’ll be able to see Io-zan itself, stark and rocky against the bright blue Okhotsk Sea a kilometer and a half below you. Between you and Io-zan is a small crater; you’ll head down into it along the lefthand rim and find a campsite nearby. Here, too, is a food locker for your goods. If you’re staying the night here, the only water will come from any snow nearby.
To get up to Io-zan, head back to the junction where you turned in to the campsite and head right, following the trail coming up from the Io River (硫黄川, Io-gawa) valley, and from there head to the summit. This last slope is quite steep and the footholds can be brittle and crumbly. If anyone is following you up, be careful not to drop a floe of rocks onto them. From the narrow summit of Io-zan, you can see out to Mt. Shiretoko-dake (知床岳, Shiretoko-dake), out to the end of the peninsula; or back along the full ridgeline that brought you here from Rausu. The views over the Okhotsk Sea are also spectacular.
In the scree field before you reach the Io River valley on the way down, it’s easy to lose your way. If visibility is poor, you have to be very careful not to accidentally enter the Ubushinotta River valley instead of the Io River valley.
As the Io River is almost totally dried up, you’ll only have to wind around a few small waterfalls on your way down. In the early season the snow in this valley will be quite unstable, so tread carefully. At about 960 meters above sea level the trail will leave the ravine and cut on the left side into a thick forest of dwarf stone pine, heading towards Shin-funkakou (新噴火口, ‘New Crater’).
The view will open up at Shin-funkakou, where you’ll have to follow paint marks on the rocks. Further along the trail you’ll come to the demolished remains of the sulfur mining pit, from which you’ll be able to see, off to the right, the Kamuiwakka Yu-no-taki (カムイワッカ湯の滝, ‘Kamuiwakka hot waterfall’) valley below. You’re close to the end of the trail.
Through a forest of Mongolian oak you’ll shortly come to a forest road. A good number of bears make their home around the trail exit, so keep your wits about you until the end. Head back towards Iwaobetsu for about 10 minutes and you’ll end up at Yu-no-taki bus stop.
The hiking season last from about late June to mid-October. Until around July there will be snow along the ridgelines and the upper parts of the mountains. The forests on the mountainsides will start changing color in mid-September. Around this time as well the first snows will fall. If climbing in October or later, there will be a decent amount of snow and winter gear is required.